Electrical utilities in the United States and throughout the world are endeavoring to make better use of their base load power plants, whether coal-fired, gas- or oil-fired or nuclear. One operating procedure has been to have a system which accepts electricity during a period of low demand (thus allowing the continuing efficient operating level of the base load plant) in a manner that electricity can be returned to the power grid during a period of high demand. One system used to achieve this desired result is pumped water storage, wherein water is pumped to a higher vertical level during off-peak hours and allowed to run hydraulic turbines during peak hours to generate electricity which is fed onto the grid. Although up to about 70% of the energy used to charge this system is recovered upon discharge, the utilization of such a system is extremely site-specific to an appropriate geographic location and is very large, requiring a large storage volume per megawatt hour, and therefore expensive, requiring a lengthy construction period.
A second such system utilizes compressed air storage, with off-peak electrical power being used to compress air which is then stored in a closed salt cavern or mine or in an aquifer. During peak hours, the compressed air is used to burn fuel (e.g. gas or oil) which is then expanded through a power turbine to produce electrical power which is fed onto the grid. Again, such a system is site-specific, is dependent upon the gas-integrity of the storage region, requires a fairly large storage volume per megawatt hour but is also quite expensive per megawatt of installed capacity.
As an alternative to storing electrical power, electrical utilities have also investigated the employment of more efficient power generation systems. One more efficient way of electrical power generation is to employ a gas or oil-fired gas turbine as a part of a combined-cycle system. In such a system, the heat rejected by the higher temperature or topping cycle is used to drive the lower temperature cycle to produce additional power and operate at a higher overall efficiency than either cycle could by itself achieve. The lower temperature cycle is referred to as the bottoming cycle, and generally all bottoming cycles have been Rankine cycles which operate on the heat rejected, for example, by the gas turbine exhaust. Steam-bottoming cycles have been the most widely used. However, there have been some small demonstration-type plants which used organic working fluids, such as pyridine or toluene, which can be heated and expanded in a manner similar to steam. U.S. Pat. No. 3,257,806 is an example of such a power plant which proposes using a main steam cycle and a variety of organic materials as the working fluid in the bottoming cycle, such as butene, butyne and isobutylene.
Although these various systems may have certain advantages in a particular situation, the electrical power-generating industry and users of electrical power have continued to search for more efficient systems.